None of them cared a whit about atheism.
Not true. You must not have read much on Marxism, which is essentially the favored ideology of all those mentioned. It specifically calls for the elimination of all religious constructs.
You see, you can't have your people effectively putting faith in the national government if they are consumed with faith in God. We can actually see this manifest in our own politics today. The religious tend to be more right-wing conservative, smaller government, less handouts to the needy, more personal responsibility... The non-religious tend to be more left-wing, bigger government, more handouts to the needy, less personal responsibility. You'd think it would be the other way around, the religious would be more about helping the needy, etc. But the religious put faith in God to take care of the needy, while the non-religious think we must rely on government taking from one class to give to another. Now, before you jump on this as a "generalization" let me add, this is not ALWAYS the case, but we still have some religious freedom in America.